Response of megalopae and early stage juveniles of the Dungeness crab, <i>Metacarcinus magister</i> Dana, 1852 (Decapoda: Brachyura: Cancridae), to a neonicotinoid pesticide

Abstract The response of the megalopa larva and the early-stage juvenile (JI) Dungeness crab, Metacarcinus magister Dana, 1852 (Cancridae), to low concentrations and short-term exposures of the neonicotinoid pesticide, imidacloprid (IMI), was examined among six laboratory tests. Experimental conditions were designed to mimic those of experimental field applications of IMI (e.g. quickly fluctuating concentrations, short time intervals) conducted as part of registration and permitting efforts for the pesticide’s use against a burrowing callianassid shrimp (Neotrypaea californiensis Dana, 1854) in an estuary in Washington state, USA. The tests found no direct mortality in juveniles and in megalopae exposed to high IMI concentrations for relatively long exposure intervals. Instead, crab entered a state of partial or full tetany that was reversible upon transfer to 0 µg l–1 IMI. Onset of partial tetany preceded on the onset of, and recovery from, full tetany. The degree of tetany and its reversal depended on both the concentration and exposure interval to IMI. Megalopae exposed to high concentrations of IMI for 4 and 18 h exhibited delayed molting to JI juveniles. The results, with effects mitigated by aspects of Dungeness crab ecology, indicated the proposed use of IMI would have had minimal impact on the estuary’s Dungeness crab population. Perhaps more importantly, the results imply that estuarine and near coastal waters worldwide should be monitored for neonicotinoid contamination and potential impacts to crabs and other crustaceans.